JP7468616B2 - Steel pipe for hydrogen gas, manufacturing method of steel pipe for hydrogen gas, pressure vessel for hydrogen gas, and manufacturing method of pressure vessel for hydrogen gas - Google Patents

Description

The present invention relates to a steel pipe for hydrogen gas, and in particular to a steel pipe for hydrogen gas that combines excellent hydrogen embrittlement resistance with high productivity and can be used extremely suitably as a component for hydrogen gas pressure vessels, etc. The present invention also relates to a method for manufacturing a steel pipe for hydrogen gas, a hydrogen gas pressure vessel, and a method for manufacturing a hydrogen gas pressure vessel.

Steel pipes are strong and relatively inexpensive, so they are used for a variety of purposes, such as piping and structural components, and in recent years, they have also been proposed for use as components that come into contact with hydrogen gas, such as hydrogen gas pressure vessels.

However, steel has a property called hydrogen embrittlement, in which hydrogen that penetrates into the steel reduces its ductility. Therefore, when steel pipes are used in contact with hydrogen gas, there is a risk of fracture due to hydrogen embrittlement. In particular, when steel pipes are used as hydrogen gas pressure vessels, not only is high internal pressure exerted on the steel pipe by the high-pressure hydrogen gas filled inside, but the steel pipe is also subjected to repeated stress as the hydrogen gas is repeatedly released and filled. This makes the problem of fatigue fracture due to hydrogen embrittlement prominent.

Therefore, various technologies have been proposed to improve the hydrogen embrittlement resistance of steel.

For example, Patent Document 1 proposes a technology for improving hydrogen embrittlement resistance by controlling the chemical composition and microstructure of steel pipes used as liners for hydrogen gas pressure vessels (pressure accumulators).

Furthermore, Patent Document 2 proposes a technology for improving hydrogen embrittlement resistance by controlling the chemical composition of low-alloy steel used in hydrogen gas pressure vessels.

Patent Document 3 proposes a technology for improving fatigue resistance properties in a high-pressure hydrogen environment by reducing the amount of coarse inclusions that serve as the starting point for fatigue fracture in low-alloy steel used in hydrogen gas pressure vessels.

JP 2018-053357 A JP 2009-275249 A JP 2018-012855 A

The conventional technologies proposed in the above Patent Documents 1 to 3 all focus on the metallurgical properties of steel to improve hydrogen embrittlement resistance. Therefore, in order to apply these technologies to steel pipes, it was necessary to adjust the composition of the steel at the steelmaking stage, and further to strictly control the microstructure of the steel and the state of inclusions by adjusting the conditions in the pipe-making process and heat treatment process. Therefore, not only were the types of steel that could be applied limited, but productivity could not be said to be excellent.

The present invention has been made in consideration of the above circumstances, and aims to provide a steel pipe for hydrogen gas that combines excellent hydrogen embrittlement resistance with high productivity and can be used extremely suitably as a component for hydrogen gas pressure vessels, etc. Another aim of the present invention is to provide a hydrogen gas pressure vessel using the above-mentioned steel pipe for hydrogen gas.

As a result of investigations conducted by the inventors to solve the above problems, they discovered that excellent resistance to hydrogen embrittlement can be obtained by an extremely simple method of controlling the angle of the polishing lines formed on the inner surface of a steel pipe within a specific range when the inner surface of the steel pipe is finish polished.

The present invention has been made based on the above findings and has the following gist:

1. A steel pipe for hydrogen gas with an inner surface that has been finished and polished,
The steel pipe for hydrogen gas, wherein the inclination angle of the polished streaks present on the inner surface with respect to the circumferential direction of the steel pipe for hydrogen gas is 0 to 30°.

2. A steel pipe for hydrogen gas as described in 1 above, in which the maximum height Rz of the inner surface is greater than 20 μm.

3. A method for manufacturing a steel pipe for hydrogen gas, in which an abrasive tool is brought into contact with the inner surface of a steel pipe as an object to be polished, and the steel pipe is moved relative to the abrasive tool to polish the inner surface of the steel pipe,
The method for manufacturing a steel pipe for hydrogen gas, wherein the inclination angle of the polished streaks formed on the inner surface of the steel pipe with respect to the circumferential direction of the steel pipe is 0 to 30°.

4. A hydrogen gas pressure vessel using the hydrogen gas steel pipe described in 1 or 2 above.

5. A method for manufacturing a hydrogen gas pressure vessel, comprising processing the hydrogen gas steel pipe described in 1 or 2 above into a hydrogen gas pressure vessel.

The steel pipe for hydrogen gas of the present invention not only has excellent hydrogen embrittlement resistance, but also has excellent productivity because it can be manufactured by an extremely simple method of controlling the angle of the polishing streaks within a specific range when finish-polishing the inner surface. Therefore, by using the steel pipe for hydrogen gas of the present invention, it is possible to efficiently and inexpensively manufacture a pressure vessel for hydrogen gas that has excellent hydrogen embrittlement resistance. Furthermore, as described below, the present invention utilizes the mechanical relationship between the stress applied to the steel pipe and the polishing streaks, and therefore, unlike conventional techniques that utilize the metallurgical properties of steel, it can be applied to various steel pipes regardless of the steel type.

1 is a graph showing the results of a fully alternating fatigue life test.

The present invention will be described in detail below. Note that the present invention is not limited to this embodiment.

[Steel pipes for hydrogen gas]
First, a steel pipe for hydrogen gas according to one embodiment of the present invention will be described. The steel pipe for hydrogen gas according to the present invention is a steel pipe for hydrogen gas whose inner surface is finish-polished, and the angle of the polishing streaks present on the inner surface with respect to the circumferential direction of the steel pipe for hydrogen gas (hereinafter referred to as the "inclination angle") is 0 to 30°.

Tilt angle: 0 to 30 degrees
When finish-polishing the inner surface of a steel pipe, polishing is generally performed by moving a grinding tool such as a grindstone relative to the steel pipe. Therefore, polishing streaks are formed on the inner surface of the steel pipe after finish-polishing according to the direction of movement during polishing. If we focus on each of these polishing streaks, we can consider them to be notch-like defects with a certain depth from the inner surface of the steel pipe in the thickness direction.

On the other hand, because steel pipes have a circular cross section and an open shape in the longitudinal direction, when pressure is applied to the steel pipe during use, the load is mainly applied in the circumferential direction of the steel pipe. For example, when a steel pipe is used as a hydrogen gas pressure vessel, filling the vessel with high-pressure hydrogen gas results in a load being applied mainly in the circumferential direction of the vessel (steel pipe).

Therefore, if the direction of the grinding streaks, which are notch-shaped defects, is close to the longitudinal direction of the steel pipe (the axial direction), a load is applied in the direction that opens the notch, and stress is concentrated at the bottom of the grinding streaks. In general environments such as in the atmosphere, such stress on the grinding streaks does not actually cause a problem, but in environments where hydrogen gas is present, the stress on the grinding streaks has a significant effect on fatigue properties due to hydrogen embrittlement. This is because dislocations, which are lattice defects in steel, increase in the areas where stress is concentrated, increasing the amount of hydrogen that penetrates, and as a result, hydrogen embrittlement becomes more pronounced. Therefore, in a hydrogen gas environment, if the direction of the grinding streaks is close to the longitudinal direction of the steel pipe (the axial direction), crack generation is promoted and fatigue properties deteriorate.

Therefore, in the present invention, in order to suppress the occurrence of cracks due to stress on the polishing streaks, the inclination angle of the polishing streaks present on the inner surface of the steel pipe with respect to the circumferential direction is set to 30° or less. If the inclination angle is 30° or less, the direction of the polishing streaks is close to the circumferential direction of the steel pipe, so even if a load is applied in the circumferential direction of the steel pipe, stress does not concentrate at the bottom of the polishing streaks, and as a result, the occurrence of cracks in a hydrogen gas environment can be suppressed.

The closer the direction of the polishing streaks is to the circumferential direction of the steel pipe, the greater the effect, so the inclination angle is preferably 25° or less, more preferably 20° or less, and even more preferably 15° or less. On the other hand, the direction of the polishing streaks may be the same as the circumferential direction of the steel pipe, and therefore the lower limit of the inclination angle is 0°.

In the present invention, as described above, the hydrogen embrittlement resistance of a steel pipe can be improved by an extremely simple method of controlling the angle of the polishing lines within a specific range.

In addition to controlling the direction of the polishing streaks, other methods of preventing the deterioration of fatigue properties due to stress concentration on the polishing streaks described above can also be considered, such as reducing the surface roughness. In fact, in the case of hydrogen gas pressure vessels, the inner surface of the vessel (steel pipe) was generally mirror-polished before use. However, in order to reduce the surface roughness to a level called mirror polishing, electrolytic polishing or the like must be performed after processing the steel pipe, which reduces productivity. In contrast, according to the present invention, excellent hydrogen embrittlement resistance can be achieved by controlling the direction of the polishing streaks, so there is no need to polish to a level called mirror polishing. Therefore, the hydrogen gas steel pipe of the present invention can be said to be highly productive from the viewpoint that processes such as mirror polishing are not required.

The inclination angle of the polishing streaks can be measured by analyzing images obtained by photographing the inner surface of the steel pipe with a microscope. Specifically, an image is first taken so that the circumferential direction of the steel pipe is parallel to the x-axis of the image. Next, the inclination from the x-axis (absolute value of the inclination angle of the polishing streaks) is measured for all polishing streaks contained in the obtained image. A frequency distribution of the inclination angles of the measured polishing streaks is created, and the average value obtained by fitting using a Gaussian function is taken as the inclination angle.

More specifically, first, six samples for observing the inner surface are taken from the steel pipe. The size of the samples may be a size that can be placed on the stage of a microscope. For example, a digital microscope HRX-01 manufactured by Hirox Co., Ltd. may be used as the microscope. The sample size may be, for example, 10 mm wall thickness x 100 mm circumferential length of the steel pipe x 100 mm longitudinal length of the steel pipe. The six samples are taken from random positions on the steel pipe. However, since the entire inner surface of the steel pipe is usually polished under the same conditions, three samples taken from each end of the steel pipe, which are easy to take, can be used for measurement.

Next, the surface of each sample (the inner surface of the steel pipe) is imaged using a microscope. The sample is placed on the stage of the microscope so that the circumferential direction of the steel pipe is parallel to the x-axis of the image. The imaging range is, for example, 3 mm x 3 mm per sample, and five or more fields of view are photographed. Note that because the inner surface of the steel pipe is curved, a three-dimensional image linking system is used to obtain the images.

The image obtained is then analyzed to measure the inclination from the x-axis (absolute value of the inclination angle of the polishing streak). At this time, continuous lines observed in the image that are 1 mm or longer in length and 0.01 mm or wider are considered to be polishing streaks. In addition, if a line is curved or bent, the line is approximated as a series of line segments each 1 mm or longer in length, and each line segment is considered to be an independent polishing streak. A frequency distribution of the inclination angles of the measured polishing streaks is created, and the average value obtained by fitting using a Gaussian function is taken as the inclination angle.

When automatic polishing is performed, all polishing streaks become approximately parallel, but the number of polishing streaks observed in one field of view becomes extremely large, so it may be difficult to measure the inclination angle of all polishing streaks. In such cases where the number of polishing streaks per field of view is extremely large, the angle calculated using the following procedure can be used as the inclination angle of the polishing streaks.

(1) First, the approximate angles of the grinding streaks present within the field of view (hereinafter referred to as "main angles") are visually determined. The main angles are in increments of 5°. In other words, the representative angles of all grinding streaks present within the field of view are selected from among 0, 5, 10, 15, 20, 25, 30, 35, 45, 50, 55, 60, 65, 70, 75, 80, 85, and 90°.

(2) Next, the inclination angle of any one of the polishing streaks present within the field of view is measured, and if the difference between the obtained inclination angle and the main angle is within 5°, the polishing streak is selected as a representative polishing streak. This procedure is repeated until the number of selected representative polishing streaks reaches 40.

(3) From the selected 40 representative polishing lines, extract 20 polishing lines so that the difference between the maximum and minimum inclination angles is smallest.

(4) A frequency distribution of the inclination angles of the 20 extracted polishing lines is created, and the average value obtained by fitting using a Gaussian function is used as the inclination angle.

Rz: 20 μm or less The roughness of the inner surface of the steel pipe for hydrogen gas of the present invention is not particularly limited and may be any roughness. However, as described above, in the steel pipe for hydrogen gas of the present invention, unlike the conventional steel pipe, the hydrogen embrittlement resistance is improved by controlling the direction of the polishing streaks, so there is no need to finish it to a smoothness level called mirror polishing. Therefore, the Rz of the inner surface of the steel pipe for hydrogen gas may be greater than 20 μm. In other words, from the viewpoint of eliminating excessive polishing steps and further improving productivity, it is preferable to make Rz greater than 20 μm. Rz is preferably 25 μm or more, and more preferably 30 μm or more.

On the other hand, there is no particular limit to the upper limit of Rz, but if the surface is excessively rough, the effect of stress concentration may become unnegligible. Therefore, Rz is preferably 300 μm or less, more preferably 200 μm or less, more preferably 180 μm or less, and even more preferably 160 μm or less.

Here, Rz is the maximum height in JIS B0601:2001. Rz can be measured by the method described in the examples.

The material of the steel pipe for hydrogen gas is not particularly limited, and any steel pipe can be used. For example, from the viewpoint of cost, it is preferable to use a steel pipe made of low alloy steel, and in particular, it is preferable to use a steel pipe made of any of chromium molybdenum steel, nickel chromium molybdenum steel, manganese chromium steel, manganese steel, and boron-added steel. Among them, from the viewpoint of achieving both material strength and cost, it is more preferable to use chromium molybdenum steel or chromium molybdenum nickel steel, which is easy to ensure hardenability.

The steel pipe is not particularly limited and may be any steel pipe manufactured by any method. The steel pipe may be, for example, a butt-welded steel pipe, a welded steel pipe, or a seamless steel pipe, but a seamless steel pipe is preferable. Seamless steel pipes are excellent in properties such as toughness, and because they have no joints, they are extremely suitable for use in applications such as high-pressure gas containers.

The size of the steel pipe is not particularly limited, and any size of steel pipe can be used. From the viewpoint of manufacturability, the outer diameter of the steel pipe is preferably 1800 mm or less, more preferably 700 mm or less, and even more preferably 500 mm or less. The outer diameter of the steel pipe is preferably 20 mm or more, more preferably 100 mm or more, and even more preferably 300 mm or more. The thickness of the steel pipe is not particularly limited, but from the viewpoint of strength, it is preferably 5 mm or more, more preferably 10 mm or more, and even more preferably 30 mm or more. The thickness of the steel pipe is preferably 100 mm or less, more preferably 80 mm or less, and even more preferably 70 mm or less.

[Method of manufacturing steel pipes for hydrogen gas]
Next, a method for producing a steel pipe for hydrogen gas according to one embodiment of the present invention will be described. The steel pipe for hydrogen gas according to the present invention can be produced by polishing the inner surface of the steel pipe so that the inclination angle of the polishing streaks satisfies the above-mentioned condition.

Specifically, the inner surface of the steel pipe to be polished can be polished by moving the steel pipe relative to the polishing tool while the polishing tool is in contact with the inner surface of the steel pipe. In this case, the polishing streaks formed on the inner surface of the steel pipe are inclined at an angle of 0 to 30° with respect to the circumferential direction of the steel pipe.

The method of movement is not particularly limited, but for example, the steel pipe may be rotated in the circumferential direction of the steel pipe relative to the grinding tool, and the steel pipe may be moved in the longitudinal direction of the steel pipe relative to the grinding tool. The rotation in the circumferential direction and the movement in the longitudinal direction may be performed independently, continuously or intermittently. For example, when the inclination angle is set to 0°, the steel pipe is ground in the circumferential direction without moving in the longitudinal direction, and then the steel pipe is moved in the longitudinal direction with grinding stopped, and the process of grinding in the circumferential direction again is repeated.

In one embodiment of the present invention, a method for manufacturing a steel pipe for hydrogen gas can include a polishing condition determination step for determining the circumferential rotation speed and the longitudinal movement speed so that the polishing streaks formed on the inner surface of the steel pipe have an inclination angle of 0 to 30° with respect to the circumferential direction of the steel pipe. The polishing conditions can usually be determined before polishing begins.

Determining the conditions so that the inclination angle is 0 to 30° means determining the conditions so that the calculated inclination angle, obtained from the circumferential rotation speed and the longitudinal movement speed, is 0 to 30°. In actual polishing, polishing streaks are formed that are roughly parallel, but some variation in the inclination angle of the polishing streaks is permitted.

The polishing tool is not particularly limited, and any polishing tool, such as various known polishing tools for finish polishing, can be used. In addition, the polishing method can be either wet polishing or dry polishing, but it is preferable to use dry polishing from the viewpoint of simplifying post-processing such as cleaning.

According to the above method, since the grinding tool is moved relative to the steel pipe while grinding, the entire inner surface of the steel pipe can be ground using a grinding tool smaller than the total area of the inner surface of the steel pipe. However, this method is an example of a suitable manufacturing method for the steel pipe for hydrogen gas of the present invention, and the steel pipe for hydrogen gas of the present invention is not limited to one manufactured by this method.

In addition, as long as the inclination angle of the polishing streaks finally obtained satisfies the above-mentioned conditions, any other treatment may be performed in addition to the polishing by the above-mentioned method. For example, after polishing by the above-mentioned method, additional polishing may be performed partially. After polishing by the above-mentioned method using an automatic polishing device, areas with uneven polishing or defects may be partially polished by hand.

The surface obtained by polishing is highly active and easily oxidized. Therefore, from the viewpoint of preventing rust, it is preferable to perform anticorrosion treatment after polishing the inner surface of the steel pipe. As the anticorrosion treatment, it is preferable to apply anticorrosion oil to the surface of the steel pipe. It is also preferable to provide covers on both ends of the steel pipe to prevent moisture from entering the pipe from the outside. Furthermore, it is also preferable to seal a desiccant inside the steel pipe.

[Hydrogen gas pressure vessel]
The steel pipe for hydrogen gas of the present invention can be used for any purpose in which it is used in a state of contact with hydrogen gas, such as a pressure vessel for hydrogen gas, a piping for hydrogen gas, etc. In particular, it can be suitably used as a steel pipe for high-pressure hydrogen gas used in a high-pressure hydrogen gas environment.

For example, when the steel pipe for hydrogen gas of the present invention is used for a hydrogen gas pressure vessel, the hydrogen gas pressure vessel can be manufactured by subjecting the steel pipe for hydrogen gas to the necessary processing. The processing is not particularly limited, but may include, for example, processing for attaching a lid to the end of the steel pipe. Processing for attaching a lid may include, for example, providing a thread for screwing the lid onto the end of the steel pipe, or forming a flange for tightening the lid with a bolt.

The steel pipe for hydrogen gas of the present invention has excellent hydrogen embrittlement resistance even with polishing streaks present, so that when used as a pressure vessel for hydrogen gas, there is no need to perform additional processing such as mirror polishing on the inner surface of the steel pipe (vessel) and it can be used as is.

In order to confirm the effects of the present invention, fatigue tests were conducted using the procedure described below to evaluate the effect of the inclination angle of the polishing grooves on the fatigue properties of the steel material.

First, a steel material made of nickel-chromium-molybdenum steel (SNCM439) was heat-treated to obtain a steel material with a tensile strength of 892 MPa, and smooth round bar test pieces with a parallel section length of 20 mm were taken from the steel material. The surface of the test piece was first mirror-polished, and then polished by rubbing it in one direction with emery paper to form polishing streaks of various inclination angles.

(Tilt angle)
The inclination angle of the polished streaks formed on the surface of the test piece was measured by the above-mentioned procedure. However, in this embodiment, a test was performed simulating a steel pipe using a round bar test piece, so the longitudinal direction of the test piece, which is the direction in which stress is applied, corresponds to the circumferential direction of the steel pipe. Therefore, the sample for measuring the inclination angle was taken from the round bar test piece so that the longitudinal direction of the test piece and the x-direction of the image were parallel. Therefore, in the following description, the angle of the polished streaks relative to the longitudinal direction of the test piece is expressed as the inclination angle. A digital microscope HRX-01 manufactured by Hirox Co., Ltd. was used to observe the polished streaks. The measurement results are shown in Table 1.

(Rz)
The maximum height Rz (JIS B0601:2001) of the test piece surface was measured using a part of the 3D measurement function implemented in a digital microscope HRX-01 manufactured by Hirox Co., Ltd. The measurement results are shown in Table 1.

(Fatigue life test)
Next, a fully alternating fatigue life test was carried out using the test specimen at room temperature in a high-pressure hydrogen gas atmosphere at a pressure of 105 MPa with a stress ratio R of -1. The stress level (loaded stress/tensile strength) in the fatigue life test was as shown in Table 1. In the test, the test frequency was 1 Hz, and the test was discontinued if no fracture occurred when the number of cycles N reached 1 million.

The results of the fatigue life test are shown in Table 1. The results for samples with Rz of around 8 μm are plotted in Figure 1. As can be seen from the results shown in Table 1 and Figure 1, steel pipes that satisfy the conditions of the present invention have an excellent fatigue life in a high-pressure hydrogen gas atmosphere.

Claims (5)

A steel pipe for hydrogen gas, the inner surface of which is finish-polished,
A steel pipe for hydrogen gas, in which grinding streaks are formed on the inner surface, the maximum height Rz of the inner surface is 6 μm or more, and the average value obtained by creating a frequency distribution of the inclination angle of the grinding streaks relative to the circumferential direction of the steel pipe for hydrogen gas and fitting it using a Gaussian function is 0 to 30°. A steel pipe for hydrogen gas according to claim 1, in which the maximum height Rz of the inner surface is greater than 20 μm. A method for manufacturing a steel pipe for hydrogen gas, comprising the steps of: bringing an abrasive tool into contact with an inner surface of a steel pipe as an object to be polished; and moving the steel pipe relative to the abrasive tool to polish the inner surface of the steel pipe,
A method for manufacturing a steel pipe for hydrogen gas, comprising forming polishing streaks on the inner surface of the steel pipe, the maximum height Rz of the inner surface being 6 μm or more, creating a frequency distribution of the inclination angles of the polishing streaks relative to the circumferential direction of the steel pipe, and fitting the frequency distribution using a Gaussian function to obtain an average value of 0 to 30°. A hydrogen gas pressure vessel using the hydrogen gas steel pipe according to claim 1 or 2. A method for producing a hydrogen gas pressure vessel, comprising processing the hydrogen gas steel pipe according to claim 1 or 2 into a hydrogen gas pressure vessel.

Images